CLARK : THE ECHINODERMS OF PERU. 325 



eighteen genera, of which five are known only from the west coast of 

 Central and South America. Ten of the species are not known outside 

 of the region included in this report, while eleven others are well-known 

 Panainic species. There can be little question that most of the starfishes 

 came to the Peruvian coast from the north. We know too little about 

 Tosia verrucosa and Parasterina obesa to be positive as to their origin, but 

 they probably came from the south. The two species of Asterina may 

 have come from the south, but it is quite as possible that they are of 

 northern stock. Odontaster is unquestionably a southern genus and 

 Henricia liyadesi certainly came up from the south. The south also 

 may be considered the home of Asterias gelatinosa, which has hardly 

 reached the Peruvian coast yet, and while there is lack of definite evi- 

 dence bearing on the point, Stichaster aurantiacus may be considered as 

 an immigrant from the south. But with these exceptions the starfishes 

 of the Peruvian coast are undoubtedly of northern (Panarnic) origin. 



In discussing the characteristic features of starfishes, a few terms require a word 

 of explanation. In many species the dorsal skeletal plates develop vertical, table- 

 like outgrowths, called paxillae; the tops (tabulae) of these paxillae are square, 

 oblong, polygonal or more or less circular, and are usually so crowded as to form 

 a second covering to the dorsal surface of the animal, between which and the true 

 surface there is a more or less considerable space, traversed vertically by the 

 columns or stalks of the paxillae ; the tabulae bear spinelets or granules along 

 their margins and on the free upper surface, and sometimes the central spinelet 

 may develop into a conspicuous spine. The sides of the rays in many starfishes 

 are defined by an upper and lower series of plates, the supero- and infero-marginals ; 

 in some starfishes the marginal plates are very large and conspicuous even in the 

 inter bra chial areas (i. e., the space on the body between the bases of adjoining 

 rays), while in others they are small and inconspicuous, and in the most highly 

 specialized forms they are completely hidden and indistinguishable ; in Luidia the 

 superomarginals appear to be wanting, but the inferomarginals are fairly conspicu- 

 ous. In most starfishes which lack paxillae, minute, finger-like papillae project 

 between the plates of the dorsal skeleton ; these are probably respiratory organs 

 and are known as papulae ; the papulae are often single, but are usually grouped 

 in the areas between the skeletal plates, and these are then referred to as papular 

 areas. The madreporic plate is usually conspicuous on the dorsal side in an inter- 

 bracliial area, but in species having paxillae it is often hard to find. The sides of 

 the furrow, on the under side of each ray, in which the tube-feet are located, are 

 guarded by a series of plates, each of which carries a row or group of spines ; the 

 plates are called the adambulacral plates and the spines make up the adambulacral 

 armature. In describing starfishes, it is customary, for brevity's sake, to let R 

 stand for the major radius, . e., from the center of til e mouth to the tip of a ray, 



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